Gap detection in single- and multiple-channel stimuli by LAURA cochlear implantees.

Gap-detection thresholds were determined for different complex patterns of electrical stimulation in four postlingually deafened LAURA cochlear implantees, to examine the nature of within- and across-channel auditory processes in more detail. Gap detectability was examined as a function of stimulus complexity (one, two, or three channels), channel distance within and across multichannel pre- and post-gap markers, stimulus asymmetry, and pulse rate. All markers roved in duration from 200 to 500 ms to ensure that subjects were not using overall stimulus duration as a cue. Gap-detection thresholds for all subjects were short (< 5 ms) when the pre- and post-gap markers stimulated the same single or multiple channels, even when the distance between simultaneously stimulated channels was large (exp. 1). For some subjects, gap detectability was more difficult in the across-channel condition, when the pre- and post-gap markers each stimulated different channels, although performance improved substantially in most subjects after extensive training (exp. 2). Additional tests with random maskers also suggest that neural interaction only affects performance mildly, and that the magnitude of the gap-detection threshold probably depends more on the subject's cognitive (in)ability to attend to the temporal gap than on the temporal acuity of their auditory system. Other stimulus conditions showed a difference in performance related to the order of the markers: gap thresholds were longer when the pre-gap marker stimulated one channel and the post-gap marker stimulated two or more channels, than vice versa (exp. 3). In addition, gap thresholds of three of the subjects increased with decreasing pulse rate from 1250 to 400 pps, a finding which may be related to the rate of the speech processing strategies used by each individual implantee (exp. 4).